My own favorite syntax(es) are TAL, TALES, and METAL (Template Attribute Language, TAL Expression Syntax, and Macro Expansion TAL). There are many implementations in various languages. The OG implementation was in Python as part of the Zope:

https://pagetemplates.readthedocs.io/en/latest/tal.html

These days I reach for Chameleon, another Python implementation:

https://chameleon.readthedocs.io/

I'm aware of at least five implementations for JS: template-tal, jstal, Distal, DomTal, ZPT-JS.